The production of heat-hardenable mixtures of substances, which are stable in storage at room temperature, from a solid finely-divided polyisocyanate, which is deactivated by partial chemical reaction on the particle surface, (preferably with aliphatic polyamines) to form a polyurea protective covering, and is suspended in at least one compound capable of reacting with isocyanate groups, for example relatively high molecular weight polyols and/or polyamines, is known. U.S. Pat. No. 4,483,974 describes the production of solid polyisocyanates having retarded reactivity which are stabilized by covering them with polyaddition products and which are formed by reactions with "amine stabilizers" (polyamines, hydrazine(s) or hydrazide compounds having terminal --CO-NH-NH.sub.2 groups). The covering is formed substantially from polyaddition products in a thin layer and in a secondary reaction of the NCO groups (from 0.1 to 25%) on the surface of the solid isocyanate particles with the amine stabilizers. The stabilization reaction produces polyisocyanate particles which react as polyisocyanates in a one-component polyurethane reactive system, i.e., as a suspension in relatively high molecular weight polyols and/or relatively high molecular weight polyamines, optionally with addition of lower molecular weight polyols or (aromatic) polyamines. The components react only above the thickening temperature, for example by heating, by destroying the covering by shearing forces, or by simply dissolving the polyadduct layer using polar solvents.
The "polyadduct" covering on the solid polyisocyanate particles varies somewhat according to the type of "amine stabilizers". Polyurea coverings are formed with polyamines, polyhydrazodicarbonamide coverings with hydrazines, and even more complex polyadducts having a plurality of variously arranged --NH-- and --CO-- groups in the polymer chain are formed with hydrazide compounds having terminal CO-NH-NH.sub.2 groups (such as dihydrazides, bis-semicarbazides, biscarbazine esters, semicarbazidehydrazides or aminohydrazides).
According to our patent application No. P 34 01 753 (corresponding to U.S. application Ser. No. 691,593, filed Jan. 15, 1985 now U.S. Pat. No. 4,574,147), it is also possible with guanidines or amidines not containing isocyanate-reactive hydrogen atoms to produce polyadduct coverings on the isocyanates which also cause retarding of the NCO reactivity. Other processes for the production of polyisocyanates having retarded reactivity are described in German Offenlegungsschriften Nos. 3,228,723, 3,228,724 and 3,228,670, and U.S. Pat. No. 4,400,497.
The stabilized polyisocyanates are preferably produced directly in suspension in polyols and/or polyamines (preferably in relatively high molecular weight polyols) optionally with addition of lower molecular weight polyols or aromatic polyamines as chain-extenders, or in relatively high molecular weight polyamines with aromatic polyamines, optionally with addition of lower molecular weight aromatic polyamines and/or lower molecular weight polyols as chain-extenders and exist in the form of useable suspensions for one-component polyurethanes.
The stabilized polyisocyanates have good storage stability in the suspending high molecular weight polyols and/or also relatively high molecular weight polyamines, even in the presence of highly effective polyurethane catalysts, and even at elevated temperatures, providing the temperatures remain below the thickening temperature. When using the stabilized polyisocyanates, reactive mixtures containing aromatic diamines as chain-extenders are eminently stable in storage and exhibit a markedly lengthened pot-life in casting systems, even in the case of using liquid aromatic polyamines.
The curing of one-component reactive mixtures may also be effected by mere heating, a rapid polyaddition reaction taking place above a certain temperature ("thickening temperature" or "thickening point"). This "thickening temperature" may be varied and a high stability in storage may be adjusted merely by varying the reaction conditions (for example, temperature during the covering reaction), choice of the reaction medium or type and quantity of stabilizer used for stabilization.
The one-component reactive system may be caused to react in a relatively low temperature range (above the thickening temperature, preferably .gtoreq.55.degree. C., more preferably from 100.degree. to 135.degree. C.) by heat hardening. It is possible to produce high quality polyurethane plastics depending on the choice of reactants.
It has been found that difficulties arise, in particular, during the production of bulky shaped articles, for example in the case of roller coatings or cylindrical shaped articles of relatively large diameter. These difficulties lead to inhomogeneous shaped articles having poor mechanical properties. Solidification of these one-component polyurethane systems above the "thickening temperature" proceeds relatively slowly as the heat from the heated molding wall is transmitted to the interior relatively slowly through the polyurethane layer being formed.
In the case of bulky shaped articles, it has surprisingly been found that a liquid core which does not solidify remains even after prolonged heating, even above the "thickening temperature". If the temperature is raised considerably above the thickening temperature, a complete thorough reaction may be possible, but shaped articles of uneven composition are generally obtained as solid, deactivated polyisocyanate has settled in the liquid core. This leads to inhomogeneous polymer zones which are highly cross-linked in some regions (brittle, hard) and under-cross-linked in other regions (soft to tacky). The large-volume shaped articles are thus unusable. In some cases bulges of liquid material are formed or streaks of differing composition are produced.